1. Field of the Invention
The present invention relates to a high frequency module that performs processing of separating a transmission signal and a reception signal from each other.
2. Description of the Related Art
Recently, cellular phones operable in a plurality of frequency bands (multibands) have been put to practical use. It is known that front-end modules for use in cellular phones conforming to the time division multiple access system and operable in a plurality of frequency bands include one in which switching between a transmission signal and a reception signal is performed by a switch circuit. Such a front-end module is called an antenna switch module or a high frequency switch module, for example. In the present patent application a unit composed of a combination of circuits that perform processing of high frequency signals and a substrate for integrating the circuits is called a high frequency module, examples of which include the foregoing front-end module. An example of substrates used in high frequency modules is a layered substrate including a plurality of dielectric layers stacked.
In a front-end module having such a layered substrate, there are cases in which some circuit components are formed using some of conductor layers located inside the layered substrate while one or more elements as one or more other circuit components are mounted on the top surface of the layered substrate. Examples of the one or more elements mounted on the top surface of the layered substrate include a surface acoustic wave (SAW) filter formed of a SAW element and allowing a reception signal to pass. In such a front-end module, there are also cases in which a plurality of external terminals for connection to external circuits are disposed on the bottom surface of the layered substrate. JP 2004-364051A discloses a high frequency module in which a SAW filter is mounted on the top surface of the layered substrate and a plurality of external terminals are disposed on the bottom surface of the layered substrate.
Recently, as disclosed in JP 2003-142981A and JP 2003-338724A, for example, it has also been proposed to use a SAW filter having two output terminals for outputting a balanced signal, as a SAW filter that allows a reception signal to pass, in a front-end module.
In the case where a SAW filter is mounted on the top surface of the layered substrate and a plurality of external terminals are disposed on the bottom surface of the layered substrate in a front-end module, it is necessary to provide one or more signal paths for connecting the SAW filter to the external terminals. As such one or more signal paths for connecting the SAW filter mounted on the top surface of the layered substrate to the external terminals disposed on the bottom surface of the layered substrate, JP 2004-364051A shows in FIG. 5 and FIG. 6 signal paths formed using through holes and conductor layers provided inside the layered substrate and terminal electrodes provided on the side surfaces of the layered substrate. Disadvantageously, however, signal paths having such a configuration are great in length because portions thereof extend to detour around the layered substrate, and consequently insertion losses of the signal paths increase. In addition, such signal paths are susceptible to interference from a circuit outside the module, because portions of the signal paths are located on the side surfaces of the layered substrate.
In the case where a SAW filter having two output terminals for outputting a balanced signal is mounted on the top surface of the layered substrate and two external terminals to be connected to the two output terminals are disposed on the bottom surface of the layered substrate, it is necessary to provide two signal paths for connecting the two output terminals to the two external terminals. Here, the distance between the two external terminals to be connected to the two output terminals of the SAW filter is not always equal to, or rather often different from the distance between the two output terminals of the SAW filter. When the distance between the two external terminals is different from the distance between the two output terminals, the lengths of the two signal paths may also differ accordingly. A difference between the lengths of the two signal paths may cause a reduction in balance of the balanced signal.